FERTIPAS: Emissions of organic FERTIlizers as Secondary Organic Aerosol Precursors

Agriculture is a major source of volatile organic compounds (VOCs), key precursors of secondary air pollutants such as ozone and aerosols. These VOCs react with atmospheric oxidants (e.g., hydroxyl radicals, ozone, nitrate radicals) to form more oxidized compounds with a low volatility that can condense to the particulate phase, driving the formation of secondary organic aerosols (SOA). SOA, a major component of atmospheric aerosols, significantly impacts air quality, climate, and human health. However, estimating SOA production remains highly uncertain due to the complexity of these processes and the diversity of precursors. The shift toward sustainable agriculture has increased the use of organic fertilizers, such as sewage sludge, compost, and animal waste. Given the vast agricultural land area, the spreading of organic fertilizers represents a potentially significant source of VOC emissions. However, their impact on the atmosphere remains poorly understood, mainly due to a lack of studies. The general aim of this work is to improve our knowledge on the impact of spreading these organic fertilizers on air quality, as a source of VOCs. Laboratory study was carried out to analyze VOC emissions from organic fertilizers (sewage sludge, compost and methanization digestate) and to assess the impact of temperature on these emissions. An experimental set-up combining a proton transfer reaction time-of-flight mass spectrometer (PTR-ToF-MS), an emission chamber and a multi-valve system was employed to assess VOC emission from three different organic fertilizers, at three temperatures (10°C, 20°C and 30°C).The analysis revealed a total emission of 118 VOCs from digestate, 99 from sewage sludge and 200 from compost. One notable observation is the perceptible diversity in the chemical composition of these three organic fertilizers. Specifically, each fertilizer presents hydrocarbon, oxygenated and nitrogenated compounds, with hydrocarbons and oxygenated compounds dominating in all three fertilizers. On the other hand, sulfur compounds are only present in sludge and compost, while digestate had a significantly higher prevalence of nitrogenated compounds. Acetone (C₃H₆O) is the most emitted compound from digestate and sewage sludge, while methanol (CH₄O) predominates in compost emissions.  In addition, compounds such as monoterpenes (C₁₀H₁₆), cresols (C₇H₈O) and phenols (C₆H₆O), known SOA precursors, were among the most emitted compounds. Secondly, most compounds showed a positive response to temperature, with some increasing linearly and others exhibiting exponential response. Conversely, very few VOCs, such as acetic acid, unexpectedly decreased with rising temperatures. The impact of temperature variations on VOC emissions and the mechanisms driving these patterns will be discussed. Lastly, the potential of organic fertilizers to form ozone through VOC emissions has been estimated for each emitted molecule. Compost had the highest ozone-forming potential followed by sewage sludge and digestate. For digestates and composts, the primary species responsible for ozone formation were hydrocarbons (63% and 60%, respectively), even though oxygenated compounds dominated their emissions. In contrast, for sewage sludge, 56% of the ozone were produced by oxygenates. The results suggested that, from the perspective of air quality, digestate may be a preferable organic fertilizer compared to compost and sewage sludge.